I'm torn here. I think it's sort of rude for parents to ask you if you'll watch their kids (for free), unless there's a big reason.
My friend apparently does not think it's rude. She does this occasionally to me. The most recent time, she asked me to watch her kids while she went grocery shopping. My grandma was visiting from out of town, and I mentioned to my friend that we (including ds) were about to have lunch. She said that her kids weren't hungry so would just play in ds's room while we ate. I gave up and said I'd watch them.
So here's the thing- I've never asked her to watch ds, even though there have been times that it would have come in handy (but definitely not necessary). I'm sure she wouldn't mind my asking, but if I do it, I'll be doing the same rude thing she does to me. It would be like condoning it even more.
But if she's not going to stop, doesn't seem to know it's rude, and it would make my life easier, maybe I should just go with the flow.
Her kids do come over and play quite often. I used to be annoyed that she'd call and ask if her kids could come play here, but now that ds is older, he and her son have their own system for which house they play at (each prefers his own house, so they take turns). I don't mind when they come and play as long as she's home and I can send them home when I want to.